Biofilms are communities of bacteria and are responsible for some chronic infections and some drug-resistant infections. Bacteria secrete various substances, including exopolysaccharides, to form the biofilm matrix. Steiner et al. studied biofilm formation by Escherichia coli, which involves the proteins encoded by the pgaABCD operon. PgaC and PgaD activity, which catalyzes the synthesis of poly-β-1,6-N-acetylglucosamine, was stimulated by bis-(3′-5′)–cyclic dimeric GMP (c-di-GMP). Bacterial strains in which c-di-GMP synthesis was compromised exhibited reduced stability of PgaD and were deficient in biofilm formation. In a bacterial two-hybrid assay, PgaC and PgaD interacted and the interaction was enhanced by increasing the synthesis of c-di-GMP and was compromised by reducing c-di-GMP production. In an in vitro β-glycosyltransferase assay with PgaC and PgaD, the addition of c-di-GMP enhanced enzymatic activity. Cross-linking assays showed that both PgaC and PgaD bound to c-di-GMP but that binding required the presence of both proteins. A genetic mutation screen identified mutants of pgaC and pgaD that conferred biofilm formation in bacteria that could not synthesize c-di-GMP. This screen identified mutations that resulted in a form of PgaD that was stable even in the absence of c-di-GMP and that when expressed as a fusion with PgaC resulted in a constitutive β-glycosyltransferase that was not stimulated by c-di-GMP. The screen also identified mutants of PgaC that stabilized PgaD in the absence of c-di-GMP. Bacteria with a targeted mutation of a specific arginine residue that is predicted to disrupt c-di-GMP binding in PgaC were unable to stabilize PgaD and could not support biofilm formation. Cross-linking experiments showed that binding of c-di-GMP to both PgaC and PgaD was lost if binding to either protein was compromised by mutation, indicating that c-di-GMP binds to both proteins. As highlighted by Hengge, the authors propose a model in which binding of c-di-GMP alters the conformation of the PgaC-PgaD complex, stimulating enzymatic activity and creating a putative pore for transport of the polysaccharide through the membrane.
S. Steiner, C. Lori, A. Boehm, U. Jenal, Allosteric activation of exopolysaccharide synthesis through cyclic di-GMP-stimulated protein-protein interaction. EMBO J. 32, 354–368 (2013). [PubMed]
R. Hengge, Novel tricks played by the second messenger c-di-GMP in bacterial biofilm formation. EMBO J. 32, 322–323 (2013). [PubMed]